Magnetic Memory Cell and Method of Fabricating Same

ABSTRACT

A magnetic memory cell in which a sensor is magnetically coupled to a magnetic media wherein the separation of the magnetic media from the sensor permits each to be magnetically optimized separate from the other, thus improving defect tolerance and minimizing the magnetic influence of neighboring cells in an array on one another. In an embodiment, the read circuitry is positioned so that no read current passes through the media during a read operation. In an alternative embodiment, processing is simplified but the read current is allowed to pass through the media.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/876,639, filed Dec. 22, 2006, and further claimsthe benefit of U.S. patent application Ser. No. 10/985,472, filed Nov.10, 2004, as well as U.S. patent application Ser. No. 10/701,319, filedNov. 4, 2003, now U.S. Pat. No. 7,173,847, and its continuation, U.S.patent application Ser. No. 11/701,322, filed Jan. 31, 2007, all ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present application is directed to solid state memory cells havingmagnetic media, and, in some aspects of the disclosed inventions, ismore particularly related to magnetic media memory cells in which thestorage media and the sensor are separated.

BACKGROUND OF THE INVENTION

Within the field of memories, there is continuing interest in findingways to increase the storage density and speed of memories. As thepersonal use of small devices gains popularity, the memory of thisequipment has to be modified to match the function and design of thesedevices. Particularly, as more and more data needs to be stored inmemory, the memory needs to have the capacity and speed to handle suchdemand.

The discovery of the phenomena of the magnetoresistive (MR) effect andthe giantmagnetoresistive (GMR) effect provided significant advancementsin the field of memory technology. These phenomena demonstrated that theresistance of multilayer thin films comprised of ferromagnetic layerssandwiching a conducting layer can change significantly depending on thedirection of an external magnetic field.

GMR is observed in magnetic metallic layered structures in which it ispossible to orient the magnetic moments of the ferromagnetic layersrelative to one another. One such type of magnetic metallic layeredstructure consists of a stack of four magnetic thin films: a freemagnetic layer, a nonmagnetic conducting layer, a magnetic pinned layerand an exchange layer. Magnetic orientation of the pinned layer is fixedand held in place by the exchange layer.

By applying an external magnetic field, the magnetic orientation of thefree layer can be changed with respect to the magnetic orientation ofthe pinned layer. The change in the magnetic orientation generates asignificant change in the resistance of the metallic layered structures.The resistance of the structure determines the logical value to bestored therein. Disk drives that are based on GMR technology use it tocontrol a sensor that responds to very small rotations of magneticorientation of the GMR free layer due to magnetization on the disk.However, the present use of this technology in disk drives requires thedisk to rotate and head to position on the track to be read, whichtypically requires on the order of 10 ms. In contrast, the access timeof the GMR technology itself is generally on the order of 3-5 ns. Thedisk drive therefore is not utilizing the full potential of fastresponse time of the GMR, which could translate into small access time.

A related phenomena is the Magnetic Tunneling Junction, as discussed inU.S. Pat. Nos. 5,835,314 and 5,629,922, where a trilayer structure of aferromagnet (“FM”)-insulator-FM exhibits increased junction resistanceacross the trilayer structure when the magnetization of the FM layers isantiparallel to one another as compared to when the magnetization of theFM layers is parallel.

Thus there is a clear need in the industry to develop fast memorieswhich can take advantage of the GMR and MTJ phenomena without requiringthe long latency associated with the rotation of a disk drive.

SUMMARY OF THE INVENTION

The present invention provides a magnetic memory cell in which, in someembodiments, an MTJ or GMR structure is implemented wherein no readcurrent passes through the media, and in which the storage media and thesensor are separated but magnetically coupled. Defect tolerance issignificantly improved over the prior art by permitting independentoptimization of the storage layer, or media layer, and a free layerwhich comprises part of the sensor together with the exchange layer. Insome embodiments of the present invention, the media layer can be two ormore times stronger magnetically than the free layer, thus permittingthe media layer to switch the free layer reliably and repeatably.

The media is typically either single or multiple layers of ferromagneticmaterials, while the sensor, or free layer, can be any CPP/CIP sensormade of multiple magnetic and non-magnetic layers capable of sensing theorientation of the media. The architecture of the present invention alsoresults in less disturbance of adjacent memory cells.

In an alternative embodiment, the read lead mask is eliminated whilestill maintaining the advantages of the first embodiment, except thatread current is permitted to pass through the media in such embodiments.

In another aspect of the invention, the shape of the write lines isconfigured to optimize the magnetic field caused when a current ispassed through the line, to minimize the amount of current needed toswitch the magnetization of the storage media, sometimes referred tohereinafter as the media. In particular, triangular shapes of conductorsare compared to rectangular conductors. Other shapes are also disclosed.

THE FIGURES

FIG. 1 illustrates in cross-sectional view a first embodiment of amagnetic memory cell configuration in accordance with the presentinvention.

FIG. 2 illustrates in cross-sectional view the magnetic memory cellstructure of FIG. 1, marked to show the current flow through the cellduring a Write.

FIG. 3 illustrates the memory cell structure of FIG. 1 during a readfunction.

FIG. 4 illustrates schematically the separation of the sensor and themedia in the present invention, which permits the media, or storagelayer, to be optimized separately from the free layer, which is part ofthe sensor.

FIG. 5 illustrates schematically the FM layers in an embodiment of theinvention, such that a manufacturing defect in the free layer is lesslikely to cause a dead bit, since the media layer can be on the order oftwo or more times stronger than the free layer, and will thus switch thefree layer such that the memory cell is functional and the data is readcorrectly.

FIG. 6 illustrates that no current passes through the media during aread.

FIG. 7 [PRIOR ART] illustrates, for prior art MRAM structures, theeffect of neighboring cells on a memory cell.

FIG. 8 illustrates, for the current invention, the reduced effect ofneighboring cells on the state of a cell.

FIGS. 9A and 9B are plots of magnetic field versus distance for a priorart MRAM cell and the present invention, respectively, and showgraphically the reduced effect of neighboring cells on the magneticfields of a cell in accordance with the invention.

FIGS. 10A and 10B are additional plots of magnetic field versus distancefor a prior art MRAM cell and the present invention, respectively, at acloser distance between a cell and its neighbors.

FIG. 11 shows a different configuration of cells in which some cells areswitched 90 degrees, but the inter-cell distance is the same as forFIGS. 10A and 10B.

FIGS. 12A and 12B show the effect of neighboring cells for a prior artMRAM device and a cell in accordance with the present invention,respectively, and again illustrates the reduced effect of neighboringcells with the current invention.

FIG. 13 shows an array of cells, with the same inter-cell distance asbefore, where some cells are switched 180 degrees.

FIGS. 14A-14B show, for a prior art MRAM cell and the current invention,respectively, the magnetic fields versus distance, again showing theimproved magnetic isolation of the current invention.

FIG. 15 illustrates in cross-sectional view an alternate embodiment ofthe present invention, in which process complexity is reduced byeliminating the read lead mask. The read and write functions remain thesame as for the embodiment of FIG. 1.

FIG. 16 illustrates, by layer, a cell in accordance with the invention,and shows the materials and thicknesses for each layer of an embodimentin accordance with one aspect of the invention.

FIG. 17 shows a detailed view of the product cross-section for a cell inaccordance with the invention.

FIGS. 18A-20C illustrate generally a fabrication process for a memorycell in accordance with the invention.

FIG. 21 is a photograph of a 1 Mb random access memory array inaccordance with the invention.

FIG. 22 illustrates a different aspect of the invention, and shows aplurality of conductor shapes for shaping and strengthening the fieldscreated by the conductors.

FIG. 23 illustrates the magnetic field resulting from one triangularshaped current carrying conductor as shown in FIG. 22.

FIG. 24 illustrates the magnetic field resulting from two currentcarrying conductor of triangular cross-section.

FIG. 25 illustrates the magnetic field generated by a conductor having arectangular cross-section.

FIG. 26 illustrates the magnetic field generated by two conductorshaving a rectangular cross-section.

FIG. 27 illustrates an alternative embodiment of the design shown inFIG. 1, and more particularly illustrates a two wire embodiment of amagnetic memory cell in which no current passes through the media bitduring a read operation.

FIG. 28 illustrates a top view of a four wire cell in accordance withthe invention.

FIG. 29 illustrates a cross-sectional view of a four wire cell inaccordance with the invention.

FIG. 30 illustrates cross-sectional and top plan views of a three wirecell and a write operation.

FIG. 31 illustrates in cross-sectional view the read operation of athree wire cell.

FIG. 32 illustrates in cross-sectional view a three wire cell.

FIG. 33 illustrates an array of three wire cells in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, an embodiment of a memory cell 100 inaccordance with the present invention is shown in cross-sectional view.A semiconductor substrate 103 has doped therein a pair of N+ channels105. A gate 110, which serves as a bottom read line, is disposed abovethe substrate and spans substantially the space between the channels105, creating a read access transistor 113. Positioned above the gate110 is an X write line 115, and above the X write line 115 is a magneticmedia bit 120. It will be appreciated by those skilled in the art thatthe X write line 115 extends substantially orthogonally into and out ofthe plane of the Figure. Proximate to and above the media bit 120 is aread lead 125, which is connected via conductors 130 to one of the N+channels 105. Positioned on the opposite side of the read lead from themedia bit is a magnetic sensor bit 135, above which are conductors whichform the Y write line and the top read line 137. The sensor 135 includesa free layer. Ground line 140 connects to the remaining N+ channel 105,although the connection could also be to Vss in some embodiments.Dielectric 145 typically is positioned in the spaces between the otherelements, as shown in FIG. 1. The media is made of either a single ormultiple layers of ferromagnetic materials. The sensor can be anyCPP/CIP sensor made of multiple layers of magnetic and non-magneticlayers which can sense the orientation of the media.

Referring next to FIG. 2, a write operation using the cell of FIG. 1 canbe better appreciated. During a write operation, the read accesstransistor 113 is turned off, and a write current WIx is applied to theX line 115 in a direction going into the paper of FIG. 2. Concurrently,a write current Wly is applied to the Y write line 137 from right toleft, as shown by the arrow, thus causing the magnetic media 120 andsensor 135 to conform to the fields generated by the write currents.

Referring next to FIG. 3, a read operation of the memory cell of FIG. 1can be better appreciated. The read access transistor 113 is turned on,while the X write line 115 is left floating, with no voltage applied. Aread current 300 is applied to top read line 137, such that read currentflows in the direction shown by the arrows through the line 137, sensor135, read lead 125, and then through conductors 130 to N+ channel 105.It will thus be appreciated that no current flows through the media 120during a read operation. Because the sensor is magnetically coupled tothe media, the state of the sensor is stable during a read while at thesame time permitting the state of the cell to be read without passing aread current through the media 120.

The isolation of the media 120 during a read operation is represented insimplified form in FIG. 4, where the read current 300 can be seen toflow through the sensor 135, into the read lead 125, and then outthrough the read access transistor 113. Again, it can be seen that nocurrent whatsoever passes through the media bit 120 during a readoperation, thus avoiding many of the problems associated with prior artMRAM devices. In addition, the separation of the media from the sensorpermits the magnetic properties of each to be optimized separately,while also providing greatly enhanced defect tolerance, as can beappreciated from FIG. 5, where the horizontal arrows show the directionof magnetization in each layer. While a horizontal orientation is shown,it will be appreciated that the present invention can also be applied tovertical recording, and a horizontal magnetization is shown only for thesake of clarity. In an embodiment, the magnetic strength of the mediacan be on the order of two times (or more) the magnetic strength of thesensor. A common problem with conventional MRAM devices is that amanufacturing defect in the free layer of the MRAM cell causes it not toswitch with a given magnetic field. Thus the conventional MRAM cell isdead. However, in the present invention, the media 120 can besufficiently greater strength than the free layer of the sensor 135,such that media will cause the free layer within the sensor to switchover a much wider tolerance than with conventional MRAM devices. Thusthe cell of the present invention is read correctly and the cell isfunctional.

The non-intrusive read aspect of the present invention can be furtherappreciated from FIG. 6, which is a further simplified view of FIG. 4.The direction of magnetization of the ferromagnetic layers is shown bythe horizontal arrows in those layers (but subject to change following awrite operation), while the direction of current flow 300 during a read,for the embodiment shown, is shown by the vertical arrows through thesensor layers and the horizontal arrow through the read lead 125.

Referring next to FIGS. 7 through 14B, an additional benefit of thedesign of the present invention can be appreciated in comparison withconvention MRAM devices. A conventional MRAM array 700 is shown in FIG.7, and the effect of neighboring cells 705 on the state of a given cell710 can be reasonably determined. By comparison, an array 800 of cellsaccording to the present invention is shown in FIG. 8, and the effect ofneighboring cells 805 on a given cell 810 can also be determined. Forthe examples shown, the material of the magnetic pieces is supermalloy,ms=8*10⁵ and all of the cells are assumed to be switched in the samedirection, as shown by the magnetization arrows. For comparison, thecalculation of the magnetic field for the cell of the present inventionwas made in the upper surface of the cell, away from the plane z=0, asthe magnetic field is zero in that plane.

The results of the determination for neighboring cells can beappreciated from FIGS. 9A (conventional MRAM) and 9B (presentinvention), with cells having the characteristics shown below FIG. 9A.The results of the comparison for closer neighboring cells can beappreciated from FIGS. 10A (conventional MRAM) and 10B (presentinvention). As can be seen, the effect of the magnetic field fromneighboring cells is much greater on the conventional MRAM cell than onthe cell of the present invention, thus providing cells of the presentinvention better stability at a given density, and also the ability toscale to greater densities than comparable MRAM cells.

When some cells are switched 90 degrees, the effect of neighboring cellschanges, as can be best appreciated from FIGS. 11, 12A (MRAM) and 12B(present invention), where the cells again have the characteristicsshown below FIG. 12A. For simplicity, FIG. 11 is used to represent theswitched array for both conventional MRAM and the present invention,although those skilled in the art will recognize that the structure ofthe array of the present invention is indicated more precisely by FIG.8. The intercell distance remains constant. The effect of neighboringcells when some of the cells are switched 180 degrees can similarly beappreciated from FIG. 13 (again representing both MRAM and the presentinvention, for simplicity, with a constant intercell distance), with thecomparison of the resulting fields shown in FIGS. 14A (MRAM) and 14B(present invention). It will be appreciated that, in the switchedconfigurations, the magnetic field of neighboring cells are the highestfor MRAM devices, while the field remains low for the anti-ferromagneticcells of the present invention.

Referring next to FIG. 15, an alternative embodiment of the invention isshown in which the read lead mask is eliminated, thus reducing processcomplexity. The embodiment of FIG. 15 maintains the separation of themedia from the sensor, but permits read current to pass through themedia, as shown by the repositioning of the read lead 125 in FIG. 15being positioned directly below the media 120, unlike the arrangementshown in FIG. 1. The embodiment of FIG. 15 otherwise operates the sameas the cell of FIG. 1, and thus the embodiment of FIG. 15 offersimproved defect tolerance and lessens the influence on and fromneighboring cells, thus permitting greater cell density and betterscalability. For the sake of simplicity, FIG. 15 uses the same referencenumerals as FIG. 1 for similar elements.

Referring next to FIG. 16, the construction of an embodiment of a memorycell such as described above is shown in greater detail. While the cellshown in FIG. 16 has the same arrangement as for FIG. 15, only theposition of and connection to the read lead 125 is changed in theembodiment of FIG. 1. The device can be manufactured as a regular arrayof memory cells which are randomly accessible. The magnetic memory arraycan be integrated between metal 2 and metal3 or metal3 and metal4 or anymetal above metal2 in any standard CMOS semiconductor process. Notethat, in at least some embodiments, the anneal step is performed beforefabricating the copper layer, to minimize copper diffusion.

A cross-sectional view of the device of FIG. 16 can be appreciated fromFIG. 17, although it will be appreciated that the particular dimensionsand materials shown are exemplary and not limiting. It will also beappreciated from FIG. 17 that a ferrous nickel layer can be included insome embodiments of the structure, to provide magnetic shielding. Aswith FIG. 16, the cell shown in FIG. 17 reflects the embodiment of FIG.15, but only the position of and connection to the read lead 125 changesbetween the embodiments of FIGS. 1 and 15.

Referring next to FIGS. 18A-18D, 19A-19G and 20A-20C, a process forfabricating the memory cell of the present invention can be betterunderstood. A conventional CMOS process is used to fabricate a siliconCMOS wafer up through metal2, after which oxide is deposited and thentrenched according to Mask 1. A seed layer deposition follows, afterwhich copper electroplating is performed, followed by CMP as shown inFIG. 19B. A maglayer stack is formed as shown in FIG. 19C, followed byan ion mill, and an Al₂O₃ deposition by PVD as shown in FIG. 19D. Anetch with mask 3 is performed, followed by M2 deposition, M2 etch, andRIE with mask 4, as shown up through FIG. 19G. A further Al₂O₃deposition is performed as shown in FIG. 20A, followed by pad definitionand pad formation in FIGS. 20B and 20C.

Reference is next made to FIG. 21, which shows an image of a 1 Mb randomaccess memory in accordance with the present invention.

From the foregoing description, it will also be appreciated that thememory structure described above is fabricated on top of the drivecircuitry, which has already been fabricated on a substrate. Thisreduces chip size for the memory device significantly as thesemiconductor circuitry lies beneath the magnetic memory array. Theprocess can be appreciated in detail from FIGS. 18A-20C, but in generalcan be seen to comprise the steps of (1) fabricating the drivecircuitry, followed by (2) fabricating interconnect metals and vias, (3)fabricating the magnetic memory, and finally, (4) fabricating pads andany other interconnects to connect to the interconnects and vias of step(2). Those skilled in the art will appreciate that the first two stepscan be performed using conventional CMOS or other semiconductorprocessing steps, and the third and fourth steps can be performed aspost-processing steps. In this manner the underlying semiconductorcircuitry and the magnetic memory array are integrated. It will beappreciated that, while the steps shown are for a magnetic memory, ingeneral the method could be applied to any device which does not requirea single crystalline semiconductor substrate. In some embodiments, theaddition of magnetic shielding may be performed after the fourthprocessing step. Because one or more of the magnetic materials and/orprocessing steps can be incompatible with CMOS and other semiconductorprocessing, careful control of the thermal budget throughout the processis preferably utilized.

Referring next to FIGS. 22-26, an entirely different aspect of theaspect of the present invention can be appreciated. In particular, ithas been discovered that, by carefully shaping the X and Y write lines,which typically occur in pairs in the memory cells discussedhereinabove, the current required to create a magnetic field sufficientto switch the magnetization of the media 120 is decreased relative towhat would be required for a conventional conductor. In particular, if asingle conductor having a triangular cross-section as shown in FIG. 22is used, a magnetic field as shown in FIG. 23 is generated. Similarly,if two conductors having triangular cross-sections as shown in FIG. 22are used, then the magnetic field of FIG. 24 results. By comparison, themagnetic field generated by a single conductor of rectangularcross-section is shown in FIG. 25, and the field generated by twoconductors of rectangular cross-section is shown in FIG. 26. It will befurther appreciated that the rhomboidal shapes shown in FIG. 22 willalso yield benefits in at least some embodiments.

Referring next to FIG. 27, a still further embodiment of the presentinvention. In the embodiment of FIG. 27, a cell having two write wiresis shown in which no read current is passed through the media. It willbe appreciated that FIG. 27 illustrates two adjacent cells whichessentially are formed as mirror images.

Referring next to FIG. 28, a top plan view of a four wire memory cellsuch illustrates the X and Y pairs of write wires and their positioningaround the media bit 120. It will be appreciated that the media bit neednot be rectangular as shown in FIG. 28, and the X and Y write lines neednot be straight. Alternatively, in some embodiments the media bit can beovoid or other shapes and the lines can follow the same basic shape asthe media bit.

Referring next to FIG. 29 a cross-section of the cell of FIG. 28 can bebetter appreciated. In particular, as shown by the read lead, it can beseen that no read current passes through the media bit in thisembodiment.

Referring next to FIGS. 30 and 31, writing to and reading from athree-wire cell in accordance with the invention can be betterappreciated. As shown by the arrangement of the media bit in FIG. 30,the X and Y write lines can be better appreciated, where current flowsin the direction of the black arrows. FIG. 31 shows a read operationfrom a three wire cell, and again with the current flowing in thedirections shown by the black arrows.

FIG. 32 shows a simplified memory cell, similar to FIG. 15, andparticularly illustrates the flow of current fpr a cell in which theread lead cause current to pass through the media bit.

FIG. 33 shows an array of memory cells in accordance with the designsshown in FIGS. 1 and 15.

Having fully described an embodiment of the invention and variousalternatives, those skilled in the art will recognize, given theteachings herein, that numerous alternatives and equivalents exist whichdo not depart from the invention. It is therefore intended that theinvention not be limited by the foregoing description.

1. A solid state magnetic memory cell comprising a magnetic media amagnetic sensor magnetically coupled to the media a read lead positionedbetween the sensor and the media at least two write lines, and asubstrate having therein drive circuitry for applying a read current tothe read lead and the sensor media without having the read current passthrough the media.